ELECTRICAL SYSTEM 387The directions of flux, current and movement in generator and motor action are given in Fig.
12.1. The coil is free to move about the axis O. The component fields are shown, the direction of the
mech-anical force, and the directions of rotation for motor and genera-tor action. The direc-tion of the
e.m.f. is such as to maintain the current in a generator and to oppose it in a motor. The action is revers-
ible: i.e. the same arrangement may act either as generator or motor.
The two-pole and four-pole machines differ considerably in con-struction. At 50 c/s. the former
run at 3000 r.p.m. and the latter at 1500. The useful range of two-pole machines has been extended to
300 MVA, and in consequence the four-pole construction is obsolete.12.2.1 Rotors
Rotors are most generally made from solid forgings of alloy steel. The forgings must be homoge-
neous and flawless. Test pieces are cut from the circumference and the ends to provide information
about the mechanical qualities and the microstructure of the material. A chemical analysis of the test
pieces is subsequently made. One of the most important examinations is the ultrasonic test, which will
discover internal faults such as cracks and fissures. This will usually render the older practice of trepanning
along the axis unnecessary.
The rotor forging is planed and milled to form the teeth. About two-thirds of the rotor pole-pitch
is slotted, leaving one-third unslotted (or slotted to a lesser depth) for the pole centre.
(a) Windings. The normal rotor winding is of silver-bearing copper. The heat developed in the
conductor's causes them to expand, while the centrifugal force presses them heavily against the slot
wedges, imposing a strong frictional resistance to expansion. Ordinary copper softens when hot, and
may be subject to plastic deformation. As a result, when the machine is stopped and the copper coals, it
contracts to a shorter length than originally. The phenomenon of copper shortening can be overcome by
preheating the rotor before starting up. With new machines the use of silver bearing copper, having a
much higher yield paint, mitigates the trouble.
Concentric multi-turn coils, accommodated in a slot number that is a multiple of four (e.g. 20, 24,
28 or 32), are used, the slot - pitch being chosen to avoid undesirable harmonics in the waveform of the
gap density. The slots are radial and the coils formed of flat strip with separators between turns. The
coils may be preformed. The insulation is usually micanite, but bonded asbestos and glass fabric have
both been used.
As much copper (or, in some cases, aluminium alloy) as possible is accommodated in the rotor
slots, the depth and width of the slots being limited by the stresses at the roots of the teeth, and by the
hoop stresses in the end retaining rings. The allowable current depends on cling and expansion. Com-
paratively high tempera-ture-rises are allowed: the hot-spot temperature may reach 1400C.
(b) Cooling. Passage for as much cooling gas as possible is provided, in small machines the main
cooling takes place at the outside cylindrical surfaces of the rotor body and retaining rings. It is usual to
have a large gap (e.g. 45 mm.) allowing for the flow of large quantities of coolant over the rotor surface.
For larger machines provision must be made for cooling the bottom of the slot, the con-ventional method
being that shown in Fig. 12.2. It is practicable to pass an appreciable volume of gas at high velocity close
to the windings, but the temperature-gradient over the slot-insulation is still a dominant factor. For the
largest ratings elaborate ventilating arrangements are necessary, and for machines of 100 MVA.